This invention relates to an apparatus and a method for using waveform shaping to reduce high frequency noise from switching (also referred to as "driving") inductive loads and, more particularly, using current slew control and voltage slew control, where "slew" refers to the slope (or rate of change) of a waveform.
An increasingly important issue in the design of electric products is high frequency electromagnetic noise. This noise can reduce product performance or cause interference with other products. Tremendous energy and cost are spent trying to eliminate the source and reduce the effects of the noise. Switching currents and voltages in electric devices such as motors, solenoids and switching regulators are sources of noise that are omnipresent. New products, such as cellular telephones, require ever lower levels of both conducted and radiated electromagnetic noise in order to operate properly. There is also increased legislation regarding containment of electromagnetic pollution. European emission and susceptibility standards are a case in point.
Sharp transitions in a waveform contain greater high frequency components. This can be shown mathematically through Fourier series and transforms. For instance, a pure sine wave has a single frequency component and a square wave has a large number of high frequency components whose amplitude decrease with frequency. Reducing high frequency content means reducing the sharpness of the transitions.
FIG. 1 shows a simple rule of thumb regarding high frequency components. In a square wave, the high frequency components decrease in magnitude from the fundamental frequency ##EQU1## at a 20 db/decade rate, where "on-time" is the time when the switch conducts current.
A waveform with slewed edges is a waveform with transitions of nearly constant slope, i.e., a ramped signal. This means that the first derivative of the waveform is controlled. Often, waveforms with slewed edges have rounded corners. In a waveform with slewed edges and a constant first derivative, the high frequency components roll off at 20 db/decade from the fundamental frequency and at 40 db/decade from a frequency equal to ##EQU2## where t.sub.slew is the transition time of the slewed edge.
Radiation from electric devices can usually be considered in terms of the dominant electromagnetic field, which is either electric or magnetic. Either type of field can introduce noise into circuits. Electric field radiation is caused by changes in voltage. Magnetic field radiation is caused by changes in current. Countermeasures often involve both reducing the source of the radiation as well as shielding receiving circuits.
Reducing electric field induced noise can be accomplished by slowing voltage transitions with elements like capacitors. Reducing coupling capacitances can lessen the strength of noise at the receiving end. This is done with metal enclosures and metal shielding of components, wires and printed circuit board traces.
Containing magnetic field induced noise is more difficult. Reducing the source strength involves slowing down current transitions. Often this is done by adding inductive elements which are usually more expensive than capacitive elements. Shielding receiving elements from magnetic field induced noise requires a special and often expensive mu-metal shield. Since magnetic field induced noise can induce current in nearby printed circuit board traces, it is often difficult to provide a complete shield.
Some switching regulator topologies integrate high frequency filter elements with power components which can help to reduce the cost. However, such topologies still often add specific components to reduce emissions.
Adding external components necessarily increases system cost so it is desirable to minimize their number. Such external components are usually added to slow the rate of change in current and/or voltage. This can be done either by diverting high frequency components after they are created (adding a filter) or by minimizing their creation. Often because the currents are large, there is an associated power loss due to the filtering.
Controlling the voltage slew (dv/dt) across an inductor is sometimes done by creating a filter. Adding capacitance to the switching node slows the voltage transition and absorbs high frequency components. However, because of the high currents involved, the capacitor may be physically large and can dissipate substantial power, reducing switching efficiency.
Switching regulators are highly desirable because of their conversion efficiency. However this efficiency comes at the expense of creating current and voltage waveforms with greater high frequency electromagnetic content. This high frequency noise couples to nearby circuitry either through conduction or radiative electromagnetic coupling (capacitive and inductive). Switching regulator designers are often forced to compromise between efficiency, noise and performance.
In a switching regulator, most electromagnetic interference is generated by: 1) abrupt changes in current through the inductor which create high frequency magnetic noise and induce changes on nearby lines; 2) changes in inductor current which create abrupt voltage changes through equivalent series resistance ("ESR") and equivalent series inductance ("ESL") in decoupling capacitors; 3) abrupt voltage changes on an output switching element which capacitively couple to ground, introducing transient currents onto power lines; and 4) turn-off of diodes which produces sharp current transients, produces high frequency magnetic noise and also may produce high frequency voltage transients through capacitor ESR.
The interfering noise is introduced into other circuitry through conduction in power and ground wires and by capacitive or magnetic radiative coupling from "hot" components to other circuitry. Typically, conducted noise is more of a problem for the lower frequencies while radiated noise is more of a problem for higher frequencies. For a switching regulator, the current in the inductor or transformer and the currents in the switching elements are usually the most troublesome sources of noise because they are the largest currents. Likewise, voltage excursions in switching regulator switches are often the greatest source of noise due to the speed of transition and connection to the power path.
In view of the foregoing, it would be desirable to provide an apparatus and a method for reducing high frequency noise components caused by switching an inductive load, without sacrificing circuit performance or adding additional components.
It would also be desirable to allow more control over the tradeoff between harmonic content and conversion efficiency.